Memory devices including ferroelectric films have attracted attention for their nonvolatile characteristics. Such memory devices are also desirable for high speed reading and writing capabilities that result from their non-destructive read out memory characteristics, which results from storing information as a polarization direction rather than as a charge on a capacitor.
Ferroelectric memory devices may comprise various components. One type of a ferroelectric random access memory (“FeRAM”) has a two transistor, two capacitor configuration similar to a DRAM. Such a device is discussed in greater detail in “Ferroelectric Memory Applications,” J. F. Scott, et al., ULTRASONIC SYMPOSIUM, 299 (1989). Another type of FeRAM is a transistor-cell type—ferroelectric field effect transistor (“FeFET”)—which stores data in ferroelectric gate transistors, and which requires no capacitor structure similar to a DRAM. The latter type of FeRAM provides the advantages over the first type of occupying less surface area and providing non-destructive readout.
Various types of FeFETs may be constructed, each having its own advantages and drawbacks. The various types may include an MFS FET, which comprises a metal layer, a ferroelectric layer, and a semiconductor layer; an MFIS FET, which comprises a metal layer, a ferroelectric layer, an insulator layer, and a semiconductor layer; an MFMS FET, which comprises a metal layer, a ferroelectric layer, a metal layer, and a semiconductor layer; and an MFMIS FET, which comprises a metal layer, a ferroelectric layer, a metal layer, an insulator layer, and a semiconductor layer.
Although FeFET devices possess many desirable characteristics, many problems have been encountered in attempts to fabricate certain types of efficient FeFET devices. For example, it is difficult to form an acceptable crystalline ferroelectric film directly on semiconductor material. Additionally, because of a chemical reaction between ferroelectric and semiconductor materials, it is difficult to have a clean interface between the ferroelectric material and the semiconductor material as ferroelectric material may diffuse into a silicon substrate. Further, there may be a problem retaining an adequate electric charge in the ferroelectric material.
In the past, these problems have been addressed with the MFMIS FET. The MFMIS FET provides a metal layer between the ferroelectric layer and semiconductor layer, thus providing a buffer layer. The composition and past methods of fabricating an MFMIS FET circuit presents problems of their own. An MFMIS FET includes an MIS capacitor in series with an MFM capacitor. For efficient low voltage operation of the MFMIS FET, the capacitance ratio between the MFM capacitor and the MIS capacitor cannot be too large. However, because the dielectric constant of ferroelectric materials is higher than that of an insulator, the MFM capacitor may have a higher capacitance than the MIS capacitor. Consequently, the MIS capacitance should be increased for efficient operation.
Possible ways to increase the MIS capacitance include the following approaches. First, the gate dielectric layer of the MIS capacitor, i.e., the insulator layer, may be thinned-down. Second, the gate dielectric of the MIS capacitor may be replaced with another material having high dielectric properties. And third, the physical area of the MIS capacitor may be made larger than that of the MFM capacitor.
In the past, the approach of increasing the physical area of the MIS capacitor led to the formation of an MFMIS FET device in the shape of an inverted-T (herein after referred to as “inverted T-shaped gate stack”), which required two photoresist masks in forming the word line. As shown in FIG. 1, an MFMIS FET device 100 in the prior art comprises a substrate 101, a doped region 102, a contact plug 103, an isolation region 104, and an inverted T-shaped gate stack 105, including a first electrode layer 106, a ferroelectric layer 107, a second electrode layer 108, an insulator layer 109, and substrate 101. MFMIS FeFET device 100 is formed using more than one word line mask—a first word line mask is used to etch second electrode layer 108 and insulator layer 109, and a second word line mask is used to etch first electrode layer 106 and ferroelectric layer 107, thus forming an MIS capacitor physically larger than an MFM capacitor. Increasing the number of word line masks increases the risk of leakage and short circuits as a result of misalignment and is incompatible with self-aligned contact etch processes commonly used for cell area reduction. To prevent leakage or a short circuit, isolation region 104 requires extra spacing between a contact plug 103 and an inverted T-shaped gate stack 105 to prevent, for example, shorts. Isolation region 104 may comprise, for example, a dielectric material.
To overcome the problems of the prior art, a MFMIS device with an inverted T-shaped gate stack formed using one word line mask and compatible with self-aligned contact processes is desired.